The following is a good primer for those uneducated in this subject. I've decided NOT to re-invent the wheel and will now use existing material I found on the net. In the long run, it will make things clearer for you, the reader, and will allow me to focus more on the VW application aspect.
I've included a .gif of the 4 stroke process to help visualize the details written below.
"I just got a Golf/Jetta 2.slow and want to make it faster. What camshaft should I get?"
How often we have seen this question posted here? I think it could quite possibly be one of the top three most frequently asked questions in the 2.0 8v engine forum. After all, the only way to get an N/A 2.0 to perform anywhere near its four cylinder peers is to drop an aftermarket cam in it. But why does installing an aftermarket camshaft have such a great effect on this engine? Or any engine for that matter? First we need to dive into how a camshaft works and what its functions are within an engine.
Before we begin:
Below are a few pieces of reference material to help visualize things that are discussed later on......
Here we have a photo description of EVERY part of a camshaft/lobe:
to help visualize the 4 stroke combustion process, refer to this:
EXHAUST SIDEINTAKE SIDE
Let's start with the functions of a camshaft. As we all know, the engine in your car is basically one big air pump. It pulls in outside air mixed with a fuel, compresses it, converts the energy from the compressed/combusted gas mixture into mechanical motion, then expels the waste gas only to repeat the process over again. And depending what rpm your engine is at, this four stroke process takes place in a fraction of a second. Ok, so that was an extremely rudimentary explanation, but you will agree that it is an air pump, right? Ok. Now, in order to get all that inbound air/fuel in and spent gasses out, there needs to be something that times all of these events. You guessed it, that something is a camshaft. What the camshaft regulates are three major events: WHEN the valves open (valve timing), HOW MUCH the valves open (lift), and HOW LONG the valves are open for (duration). Out of those three, the two that are most critical are the when and the how long . Changing these two events could take your car from a docile, smooth idling *****cat to something that sounds like a 1940 John Deere farm tractor.
Moving forward, I will break everything up into two parts. Part 1 will consist of the physics behind camshaft functions. In part 2, I will do my best to guide you into taking whats written in Part 1 and apply in terms for our specific engines and to help you make an educated decision on your next camshaft purchase.
Considerable information has been recorded about numerous aspects of the four stroke internal combustion engine. Nevertheless, only a small percentage of people really understand how it works and even fewer still know how to modify an engine to suit their needs. I will try to simplify this complex subject by discussing some basic principles that may be overlooked or misunderstood by the average person. First, it is very important to understand the relationship between piston travel directions and valve timing events. The reason this relationship is important is because it is one of the few things that is relatively easy to adjust/change. The camshaft which opens and closes the valves makes ONE complete revolution (360 degrees) while the crankshaft moving the piston up and down the cylinder rotates TWICE (720 degrees). Camshaft timing is usually expressed in terms of crankshaft degrees relative to the piston location in the cylinder. That is, relative to Top Dead Center (TDC) and Bottom Dead Center (BDC), respectively. Note that during the four strokes of a piston in an internal combustion engine the crankshaft will rotate 720 degrees and the piston will be at each TDC and BDC twice.
THE FIRST STROKE.
Starting at TDC, the piston starts from zero velocity and moves down the cylinder during the intake stroke; first picking up speed and then slowing down again when it reaches the bottom of the stroke. As the piston moves down the cylinder, the intake valve is opening. Some air/gas mixture starts to flow into the cylinder as the valve opens, but the greatest gulp comes when the pressure differential is the greatest. This occurs when the piston reaches its maximum velocity somewhere between 70 to 80 degrees ATDC. What governs piston velocity is the stroke, rod length, RPM, and piston pin off-set. The maximum piston speed of the engine is then limited by the resistance to gas flow of the engine and/or the stresses due to the inertia of the moving parts. You must be wondering why I'm talking about piston velocity during the first stroke.
FACT ONE: Volumetric efficiency is directly related to piston velocity!
Volumetric efficiency is a measure of the effectiveness of an engine's intake system and there are about 200 miles of air above the engine just waiting to fill the cylinder with 14.7 psi at sea level. The intake valve is almost closed as the piston reaches BDC, but it does not close completely until after BDC, when the piston is on its way back up the cylinder. The reason for this is because the incoming air/fuel mixture still has momentum even though the piston has slowed way down. We are now starting,
THE SECOND STROKE.
The piston compresses the air/fuel mixture to a high enough pressure and temperature to permit spark plug ignition. We hope that this results in a CONTROLLED BURN, rather than an explosion (detonation), that produces POWER and moves the piston down for,
THE THIRD STROKE.
Power is produced while the gases in the cylinder expand and cool. In most instances, the gases are at a relatively low pressure by the time the crankshaft reaches 90 degrees After Top Dead Center (ATDC), so we can safely open the exhaust valve Before Bottom Dead Center (BBDC) to take advantage of blow-down. Otherwise, the piston would have to push ALL the exhaust out. When the piston reaches BDC we begin,
THE FOURTH STROKE.
The exhaust valve is opening at a fairly rapid rate, the piston is going up, and if the exhaust valve is not open a lot by the time the piston reaches maximum velocity, there will be resistance in the cylinder caused by excessive exhaust gas pressure. This produces conditions which are referred to as pumping losses. As the piston reaches the top of the cylinder, the end of the fourth stroke, you will see the exhaust valve is almost closed, but, lo and behold, the intake valve is just beginning to rise off the seat! At TDC at the end of the fourth stroke, both the intake and exhaust valves are open just a little. For this reason, this part of the stroke is called the OVERLAP PERIOD.
During the overlap period you will often find that both valves will be open an equal amount. This condition is referred to as SPLIT OVERLAP. On standard engines, the valves are only open together for 15 - 30 degrees of crankshaft rotation. In a race engine operating at 5 - 7000 RPM, you will find the overlap period to be in the neighborhood of 60 - 100 degrees (which also translates to more total duration)! As you might expect, with this much overlap the low speed running is very poor and a lot of the intake charge goes right out the exhaust pipe.
Let us review the four strokes again and add some timing events to calculate the total valve duration. For illustrative purposes, we can discuss a good street cam with a 268 degree duration and 108 degree lobe centers. (The lobe center angle is the angle in camshaft degrees between full intake cam lift and full exhaust cam lift). As we discussed above, at the end of the fourth stroke both valves are open and the next stroke is the intake stroke. Referring to the engine .gif up top, we see that the intake valve began to open at 26 degrees BTDC. The piston moves down the cylinder after the crankshaft passes TDC, and the valve reaches full lift at 108 degrees ATDC (lobe center). Note also that the intake valve is still open when the piston reaches BDC. We can start to add things up now. The crankshaft has rotated 180 degrees from TDC to BDC on the first stroke and the intake valve opened 26 degrees BTDC, so the total crankshaft rotation so far is 26 + 180 = 206 degrees. We started with a 268 degree camshaft so that tells us when the intake valve will close: 268 - 206 = 62 degrees ABDC. Note that even though the second stroke is the compression stroke, we see that it starts while the intake valve is still open!
FACT TWO: In the lower RPM range, the engine does not have any compression until the intake valve closes. As the engine speed increases, there is a ram or inertia effect which begins compression progressively sooner with engine speed.
Now, we compress the air/fuel mixture and ignite it at the proper time in order to maximize the push down on the power stroke, or stroke three. Remember, I said most of the cylinder pressure is gone by 90 degrees ATDC, and you can see that with our 268 degree cam, that the exhaust valve begins to open 62 degrees BBDC, that is, before the exhaust stroke actually begins. So adding again, we have 62 + 180 (stroke four) = 242 degrees. Thus at TDC at the end of the exhaust stroke, the intake valve has opened but the exhaust has not closed. The exhaust valve remains open for 268 - 242 = 26 degrees ATDC. With the intake valve opening at 26 degrees BTDC and the exhaust closing at 26 degrees ATDC we have a total of 52 degrees of overlap.
Now, with the basics down, we can start discussing duration, lift, lobe centers, compression, and cylinder flow.
VALVE TIMING EVENTS - ORDER OF IMPORTANCE
Let us now take the four valve timing events and put them in order of importance. The LEAST important is the exhaust valve opening. It could open anywhere from 50 degrees to 90 degrees BBDC. If it opens late, close to the bottom, you will take advantage of the expansion, or power, stroke and it will be easier to pass a smog test, but you will pay for it with pumping losses by not having enough time to let the cylinder blow-down. You must let the residual gas start out of the exhaust valve early enough so that the piston will not have to work so hard to push it out. Opening the exhaust valve earlier will give the engine a longer blow-down period which will reduce pumping losses. But, if you are only interested in low speed operation, say up to 4000 RPM, you can open the exhaust valve later.
The next least important timing point is the exhaust valve closing. If it closes early, say around 15 degrees ATDC, you will have a short valve overlap period. Less overlap makes it easier to pass the smog test, but it does not help power at the higher engine speeds. Closing the exhaust valve later, in the vicinity of 40 degrees ATDC, will mean a longer valve overlap period and a lot more intake charge dilution that will translate into poor low-speed operation. Some compromise must clearly be made to determine just how much overlap one needs to use. Many factors such as idle quality, low speed throttle response, fuel economy, port size, and combustion chamber design must be considered in making this choice.
A somewhat more important timing event is the intake valve opening. Early opening allows for a greater valve overlap period and adds to poor response at low engine speeds. Now, for the high performance enthusiast, low engine speed could mean 3000 RPM, but I would not consider such an engine as appropriate for normal street use! If you are not concerned about passing the smog test, then early intake valve opening will help the power output of the engine. That is, earlier valve opening will have the valve open further when the piston reaches maximum velocity and that, in turn, will increase volumetric efficiency.
I must stop now and ask you a question about your engine. If a stock ABA head does not flow much air above 0.448" of valve lift, and it is possible to have the intake valve open that much by the time the piston reaches maximum velocity, why do some people think they need a lift higher than that???
Now, the last timing event is the most important, and the most critical to engine performance - THE CLOSING OF THE INTAKE VALVE. This event governs both the engine's RPM range and its effective compression ratio. If the intake valve closes early, say about 50 degrees ABDC, then it limits how much air/fuel mixture can enter the cylinder. Such an early closing will provide very nice low speed engine operation, but at the same time it limits the ultimate power output as well as RPM. Another problem with early intake valve closing that most people do not consider is that if you have a high compression engine, say 10:1 or higher, you will have more pumping loss trying to compress the mixture. This might even lead to head gasket and/or piston failure! These observations suggest that if you close the intake valve later the cylinder will have more time to take in more air/fuel and the RPM will move up. That seems simple enough, doesn't it? The later the intake valve closes the higher the RPM and therefore the more power, MAYBE? It turns out that if the intake valve closes past 75 degrees ABDC, you could lose most of your low-speed torque and if your static compression ratio is only 8:1, the engine will not be able to reach its horsepower potential. This should give you a better understanding of why the intake valve closing is the most important timing event.
CAM SELECTION REQUIREMENTS
So, now you ask, "What do I need to know to make a proper camshaft selection for my particular application?" The list is long. First of all, in what RPM range will you want power: 1-4000 RPM, 3-6000 RPM, 5-8000 RPM, etc.? What is the size of the engine? What are the bore and stroke dimensions? How long is the center-to-center distance on the connecting rod? How much piston pin offset is there? What is the static compression ratio? In the cylinder head, what is the maximum air flow (in cubic feet per minute or CFM) in the intake track with the intake manifold and throttle body installed? At what valve lift does the air flow level out on both the intake and exhaust valves? What is the percentage of air flow of the exhaust versus the intake? What are the valve sizes? What are the lengths and sizes of the intake and exhaust systems? Once you have this data, you should be able to make a logical cam choice; but sometimes you might have to face the reality that your basic engine parameters are wrong for the RPM range you are after. How can a layperson look in a cam catalog and make an intelligent choice? First the parts supplier must supply the proper information in order to help the customer choose the right camshaft for his/her application. But, in addition, you need to be prepared with the right information about your engine and what you ultimately want to be driving.
CYLINDER HEAD FLOW BASICS
Let us now review some basic cylinder head data that one must consider before selecting a camshaft. Most people will agree with the statement that larger valves are required for more power. But now we need to ask several questions. What happens to the volumetric flow rate (in CFM) when valve sizes are increased? What about the port velocities, both intake and exhaust? How are the exhaust and intake flows effected? IS BIGGER REALLY BETTER? When you are dealing with a stock cam, say 250 degrees duration, it does indeed help to increase the valve size to get more flow through the engine. Low to mid-lift flow is very important on the exhaust valve and mid-lift to full lift flow is very important on the intake valve. Some engines respond to increasing the exhaust flow so that it almost matches the intake flow. Based on valve diameters, you will find that the exhaust flow is about 80% of the intake flow in your typical engine. Design guidelines developed by the Society of Automotive Engineers (SAE) suggest that the exhaust flow should be 75-80% of the intake. I prefer to be in the 80-85% range and port the head to achieve about 75-80% exhaust CFM flow compared to intake CFM flow. When using a stock cam, you can get good results even at exhaust/intake ratios of 90-95%. Such high ratios will also work in drag racing applications where the engine is intended to operate at wide open throttle (WOT) conditions. However, when a camshaft with more duration is installed in a "hot" street, auto cross, or road racing engine, a 90-95% exhaust/intake flow will over scavenge the cylinder resulting in wasted fuel and an undesirable reduction in torque.
COMPRESSION AND DURATION
Just about any engine would benefit from a prepared cylinder head, a good exhaust system (with a relatively small diameter for street use), and maybe a little larger throttle body bore. As you increase the RPM band, you'll need to increase the compression ratio and add some more duration to the cam. The more duration you add, the more compression you'll need and that combination will increase the upper mid-range and top-end power. It is very important to keep your combination balanced.